Cracked Rabbit Gaming has produced a three-part tutorial on creating autotiles for RMVXA (which follows the same principles in RMVX and RMXP).
They are quite long, but they’re comprehensive and are well worth watching through.
How to Create Autotiles #1 – Color-Coded Anatomy
How to Create Autotiles #2 – Creating Seamless Tiles in Photoshop
How to Create Autotiles #3 – Creating Seamless Autotiles!
So, armed with this knowledge, happy creating!